Digital signals have been widely used in various wired and wireless communications, and in recent years, a communication system has also been researched, which directly transmits the digital signals without using a carrier through a human body and at close range.
In general, in order to ensure stable signal transmission and robustness of a signal under a channel environment with noise and an interference signal, a method of spreading a digital signal having information to be transmitted in predetermined multiple is used. A digital carrier or a spreading code is used to spread the digital signal, and in a CDMA system which is widely known, orthogonality is provided between the spreading codes to discriminate a plurality of channels using the same frequency band.
However, since the above spread signal occupies a bandwidth wider than an original signal, frequency use efficiency deteriorates and a data transmission rate also decreases under the same clock.